This invention relates in general to the field of protective coatings and, more particularly, to a corrosion resistant coating system and method.
Cadmium plate and coatings containing cadmium are generally used to protect steel, ferrous, non-ferrous, and other materials or assemblies from corrosion. For example, because cadmium is anodic to steel or iron, the ferrous material is protected at the substrate level even if the substrate becomes nicked or scratched. Cadmium is generally applied as a thin coating on the order of one mil or less to components such as fasteners, fittings, plates or other types of components. For example, cadmium plating or a coating containing cadmium is often applied to threaded fasteners, such as bolts, to reduce the galvanic effects between the fastener and assembled components that are constructed from dissimilar materials. Thus, for example, in assembling aluminum components using steel fasteners, cadmium plating may be applied to the steel fasteners to reduce the galvanic effects between the steel fasteners and the assembled aluminum components.
Cadmium plating and/or coatings containing cadmium are also used to protect components from the effects of corrosion and corrosion related cracks and hydrogen embrittlement. For example, environmental exposure of steel components may result in hydrogen absorption by the steel components, thereby causing hydrogen stress cracks. Thus, the cadmium plating substantially prevents hydrogen absorption by the component, thereby minimizing the likelihood of hydrogen stress cracks.
Cadmium plate and coatings containing cadmium, however, suffer several disadvantages. For example, acute toxicity is a hazard associated with the use of cadmium in plating and/or coating operations. For example, poisoning may result from the ingestion of cadmium dissolved from containers. Additionally, inhalation of dust or fumes of cadmium salts and cadmium oxide may result when cadmium plated parts are heated. Cadmium is also associated with being a human carcinogen, a teratogen, and may cause reproductive damage in humans and animals. Cadmium also may be released into the environment as a result of leaching of landfills or from waste streams associated with industrial applications. Because cadmium is subject to EPA regulations, the costs and liabilities associated with cadmium use is generally prohibitive.
The present invention provides a corrosion resistant coating system and method that addresses shortcomings of prior coatings and methods. In particular, a multi-layer coating is provided that substantially reduces corrosion and hydrogen embrittlement of substrate materials as an alternative and/or substitute for cadmium.
According to one embodiment of the present invention, a method for producing a corrosion-resistant article includes providing a substrate and applying a multi-layer corrosion resistant coating to the substrate. Applying the multi-layer corrosion resistant coating includes applying a first layer to the substrate and applying a second layer over the first layer. The first layer includes a metal substantially galvanically similar to the substrate. The second layer includes a metal substantially anodic to the substrate.
According to another embodiment of the present invention, a corrosion resistant article includes a substrate and a multi-layer resistant coating disposed over the substrate. The coating is operable to resist corrosion and hydrogen embrittlement of the substrate. The multi-layer resistant coating includes a first layer comprising a metal or alloy of metals galvanically similar to the substrate. The multi-layer resistant coating also includes a second layer disposed over the first layer. The second layer includes a metal or alloy of metals anodic to the substrate. The coating also includes a corrosion-resistant interface layer disposed at the boundary of the first and second layers. The interface layer may be formed by diffusing a portion of first or second layer into a portion of the opposite layer.
Technical advantages of the present invention include providing an improved corrosion and hydrogen embrittlement resistant coating for metal substrates. In particular, a first layer galvanically similar to the substrate protects the substrate from the effects of corrosion and hydrogen embrittlement. A second layer selected from a group of elements anodic to the substrate is applied over the first layer to provide a coating that is similar on the galvanic series for dissimilar metal protection.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims.